Alternatively, if you only wish him to no longer have access to your database you can make his User Inactive. He wont be able to log in, but it will retain all the records as is. To do this you will need to be logged in with the Administrator User of ACT. Under the Tools menu select Manage Users. (If you are not logged in as Admin User this option will be greyed out)

Select the User you wish to make inactive then click Edit User Information. Click Next till you get to the Specify Access part of the wizard and select Inactive. Then press finish. The user will no longer be able to log into ACT.